At 02:44 PM 1/25/2006, David Minkler wrote:
>Rob,
>
>What's keeping the non-inverting input (of the TL072) from drifting off
>into a rail?  I didn't hear anything in your description that sounded
>like a DC path.
>
>Dave
>
>Rob Robson wrote:
>
> >The op amp non-inverting input is coupled to the output of an 
> LM1972 digital
> >attenuator via a 1uf electrolytic.  This part of the circuit is 
> identical to
> >one that has been in use flawlessly for years.  The inverting input is tied
> >directly to the output, which feeds a 5532 summing amp via a 0.1uF ceramic
> >cap.

I think that Dave nailed it.

Try adding a 1M0 resistor from the non-inverting input (+) input of 
the opamp to ground.

dwayne

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